It is known to provide exhaust hoods above cooking areas to remove smoke, steam and odors from kitchens. The high temperature air exhausted from a cooking area can be laden heavily with grease and other contaminants. It is desirable to remove the grease and other contaminants before the air is released into the atmosphere so that clean air is exhausted. Further, it is desirable to remove a substantial portion of such contaminants early in the exhaust system so that only a minimal amount of contaminants deposit on the ducting near the exhaust system entrance, hence requiring less frequent cleaning.
A variety of different filters, screens and contaminant removal devices are known for kitchen exhaust hoods. Known grease filters, screens and capturing structures include serpentine paths for collecting grease on the surfaces thereof, electrostatic precipitators for removing grease particles, stainless steel mesh for trapping the particles and ultraviolet scrubbers for oxidizing small grease particles. While these and other known grease removal structures have been used commonly, frequent cleaning is required, and the costs are high. In general, known structures are effective only for removing large particles (10 microns and larger), and do not work well for capturing small grease particles.
Impact classifiers, or impactors, are known and can be designed with relatively high specificity for classification of particles in air streams. In an impactor, an air stream is accelerated and directed at a surface. Inertia of particles heavier than air carries the particles against or close to an impact surface. However, the air stream having negligible inertia turns more quickly. The particles of greater mass deviate from the air stream and hit the impact surface. The influence of the surface on the deflection path that each particle follows depends on the mass of the particle. Nozzle size, air velocity, the distance from the nozzle outlet to the impact plate and other dimensional characteristics can be used to alter the performance of the device. A known formula applying the Stokes number can be used to calculate the nozzle characteristics required to collect or remove particles above and at a specific mass. If the particles being processed are of substantially similar densities, the separation becomes one related to size of the particles. This specific particle size is known as the cut off size for the performance of the impactor. Ideally, particles of the cut off size and larger are removed efficiently from the air stream, and particles smaller than the cut off size are carried through the impactor by the air stream. Generally, smaller particles can be separated by decreasing the nozzle size and increasing the air stream velocity.
While known for use in classifying relatively solid particles, wherein a supply of the particles is processed through the impactor, impactors are not known to be used as filters, for removing contaminants from air streams. Applying impactor concepts to the collection of sticky substances such as grease and cooking residues in a substantially continuous process requires also substantially continuous removal of the contaminants collected by the filter to prevent re-entrainment of the particles in the exiting air stream, or at least isolation of the collected contaminants from the exiting air stream.